For acrylic sheet (odd ends), tools and supplies I've always done well at Tap Plastics:
http://www.tapplastics.com/
For Bay Area folks there is one right off Mowry in Frement that has an odd-n-ends bin where you can get 1/8,3/8, 1/4 and 1/2 clear, blue and black acrylic pieces. 8^) You would be surprised what I've patched and built using said pieces and thick acrylic glue.
PPS Don't forget Toyo 4", 30db, 75 CFS fans from Fry's Electronics for $16 a piece. What a steal.
Scott